Low voltage analog-to-digital converters with internal reference voltage and offset

ABSTRACT

An A-to-D converter system having programmed reference signal levels using only supply signal provided by a power supply is disclosed. The converter system includes a comparator configured to provide comparison of an analog input signal with an adjustable reference level. The converter system also includes a logic circuit and an adjustable capacitor.

BACKGROUND

The present disclosure generally relates to analog-to-digitalconverters, and specifically to establishing internal reference voltageand offset in such converters.

In typical analog-to-digital (A-to-D) conversion, reference voltagelevels are used to generate a digital representation of an analog inputsignal. Dynamic range/signal resolution is often maximized when theexpected range of the analog input signal matches the reference voltagelevel.

FIG. 1 shows one type of A-to-D converter 100 that uses a techniqueknown as successive approximation. The operation of this A-to-Dconverter 100 is analogous to weighing an unknown object on a laboratorybalance scale as {fraction (1, 1/2, 1/4, 1/8)}, . . . 1/n standardweight units. The largest weight is placed on the balance pan first; ifit does not tip, the weight is left on and the next largest weight isadded. If the balance does tip, the weight is removed and the next oneadded. The same procedure is used for the next largest weight and so ondown to the smallest. After the n-th standard weight has been tried anda decision made, the weighing is finished. The total of the standardweights remaining on the balance is the closest possible approximationto the unknown weight. This weighing logic is implemented as a D-to-Aconverter 102 in FIG. 1.

One embodiment of the successive approximation A-to-D converter 200 isillustrated in FIG. 2. A bank of capacitors 202 and switches 204implement the weighing logic 201 with successively smaller sizecapacitors. A capacitor of size 2^(n−1)*C represents themost-significant bit (MSB) while a capacitor of size C represents theleast-significant bit (LSB). The value n is the number of binary bits inan A-to-D converter 200. Maximum capacitance provided at the inputsignal node 214 is (2^(n)−1)*C=C+ . . . +2 ^(n−2) *C+2^(n−1)*C. This isequivalent to a digital value of all ones. Therefore, the LSB voltage is$V_{LSB} = {\frac{V_{MAX}}{C_{MAX}} = {\frac{V_{REF}}{\left( {2^{n} - 1} \right)*C}.}}$

An input signal (V_(IN)) 206 is sampled onto the bank of capacitors 202and a comparator 208. Initially, the bottom plates of the capacitors 202are grounded. During the conversion process, the bottom plates of thecapacitors 202 are successively connected to the reference voltage(V_(REF)) 210. Corresponding bits are derived and stored in latches 212.

A reference voltage level is generally adjusted and programmed to theinput signal level. Since this reference voltage level is often adjustedto the full voltage swing of the input signal, the reference voltagemust either be supplied to the A-to-D converter 200 from off-chip orgenerated on-chip using reference circuits.

SUMMARY

The present application defines an A-to-D converter system havingprogrammed reference signal levels using only supply signal provided bya power supply.

The converter system includes a comparator configured to providecomparison of an analog input signal with an adjustable reference level.The converter system also includes a logic circuit and an adjustablecapacitor.

The logic circuit is coupled to the comparator, and has successivelysmaller size capacitors. Each capacitor is connected to at least oneswitch. The switch is configured to successively connect each capacitorto different levels of the supply signal. The adjustable capacitor isalso coupled to the comparator, and is configured to provide additionalcapacitance. The additional capacitance reduces full swing of theadjustable reference level to enable the logic circuit to operate withthe supply signal.

The present application also defines a method of converting analogsignal to digital signal. The method includes adjusting a referencecapacitor at an input signal node to appropriately reduce full swing ofa reference level. Conversion capacitors are selectively connected to asupply signal to program the reference level. The method also includescomparing an input signal to the programmed reference level, and readinga digital output value into latches if the comparison results in amatch.

BRIEF DESCRIPTION OF THE DRAWINGS

Different aspects of the disclosure will be described in reference tothe accompanying drawings wherein:

FIG. 1 is a block diagram of a successive approximation A-to-Dconverter;

FIG. 2 is a detailed block diagram of a successive approximation A-to-Dconverter;

FIG. 3 is a schematic diagram of an A-to-D converter system according toone aspect;

FIG. 4 is a flowchart of an A-to-D conversion process according toanother aspect;

FIG. 5 shows an example of a CMOS image sensor integrated circuit havingthe A-to-D converter of the present invention;

FIG. 6 is a block diagram of a pixel array and associated readoutcircuit and an A-to-D converter; and

FIG. 7 is a schematic diagram of an A-to-D converter system according toa further aspect of the present invention.

DETAILED DESCRIPTION

The present application defines an A-to-D converter system that providesan efficient solution to the problem of supplying the reference voltage.In one aspect, the solution considers implementation of the A-to-Dconverter in compact micro-power level circuits.

For example, an array of A-to-D converters is used in CMOS imagesensors. These sensors can include active pixel sensors (APS) andcharge-coupled devices (CCD). The image sensor is arranged into an arrayof column pixels and row pixels. Each pixel collects electrical chargewhen exposed to light. Control signals provided to the pixelsperiodically enable the controllers to transfer the collected charge tothe array of A-to-D converters. The collected charge is converted todigital data and stored in the column-parallel latches.

Since the available chip area and power is limited in column parallelcircuits, it is advantageous to provide a substantially compact designwhere the reference voltage uses the existing supply voltage. Further,by adjusting the total capacitance of the binary-weighted conversioncapacitors, the effective reference voltage can be changed.

A schematic diagram of an embodiment of the A-to-D converter system 300is shown in FIG. 3. The converter system 300 eliminates the need for theinternally-generated or externally-supplied reference voltage 210 byusing the rail supply voltage (V_(DD)) 304. The converter system 300allows the capacitors 302 to use the existing supply voltage. 304 byproviding an adjustable reference capacitor (C_(REF)) 308 at thepositive input signal node 306. Initially, the bottom plates of thecapacitors 302 are grounded. During the conversion process, the bottomplates of the capacitors 302 are successively connected to the supplyvoltage 304.

The adjustable reference capacitor. 308 provides additional capacitanceat the positive input signal node 306. Thus, the maximum capacitance atthe positive input signal node 306 increases to

(2^(n) −1)*C +C_(REF).  (2)

The least-significant bit (LSB) voltage is equal to $\begin{matrix}{V_{LSB} = {\frac{V_{MAX}}{C_{MAX}} = {\frac{V_{DD}}{{\left( {2^{n} - 1} \right)*C} + C_{REF}}.}}} & (3)\end{matrix}$

In one example, if the value of C_(REF) 308 is adjusted to equal thetotal capacitance ((2^(n) −1)*C) of the conversion capacitors 302, themaximum capacitance at the positive input signal node 306 becomes 2 *(2^(n)−1)*C. Therefore, the effective reference level of the A-to-Dconverter 300 that is required to match the input signal swing 310 isreduced to one-half that of the conventional A-to-D converter 200.Further, the actual capacitance value of C_(REF) 308 can be adjusted toreduce the effective reference voltage level by any amount within sometolerance value.

In some embodiments, the metal-oxide silicon field-effect transistor(MOSFET) switches 312 are appropriately modified for a low-voltageapplication when the supply voltage 304 is used in place of theinternally-generated or is externally-supplied reference voltage 210.For example, when the supply voltage 304 is about 1.2 volts and thethreshold voltages of the switches 312 are more than 0.6 volts, then-channel switches cannot effectively pass voltages close to one-half ofthe supply voltage 304. Therefore, the p-channel MOSFET switches 312 areused to connect the bottom plates of the conversion capacitors 302 tothe supply voltage 304.

FIG. 4 shows a flowchart of an A-to-D conversion process. According toan illustrated embodiment, the conversion process uses the supplyvoltage instead of the externally-supplied or internally-generatedreference voltage.

At step 400, a reference capacitor at the positive input signal node isadjusted to appropriately reduce an effective reference signal level.Once the reference capacitance is adjusted to some optimum value, theconversion capacitors are selectively connected to the supply voltage atstep 402. The selective connection programs the reference signal level.At step 404, the input signal is compared to the programmed referencesignal level. If the comparison match is found (step 406), a digitaloutput value is read out from the latches at step 408.

Although the above-described solution slightly increases the dynamicpower consumption in an A-to-D converter, the solution reduces theoverall system power consumption. This solution is especially beneficialto low- voltage, low-power CMOS imagers because the supply voltage(approximately 1.2 to 1.5 volts) is close to the required referencevoltage (approximately 1.0 volt.). Other advantages include overallcircuit simplification and no decoupling capacitors that are required tostabilize the reference voltage.

FIG. 5 shows an example of a CMOS image sensor integrated circuit chip500. The chip 500 includes an array of active pixel sensors 502 and acontroller 504. The controller 504 provides timing and control signalsto enable read out of signals stored in the pixels. For someembodiments, arrays can have dimensions of 128×128 or some larger numberof pixels. However, in general, the size of the array 502 will depend onthe particular implementation. The image array 502 is read out a row ata time using a column-parallel readout architecture. The controller 504selects a particular row of pixels in the array 502 by controlling theoperation of the vertical addressing circuit 506 and row drivers 508.Charge signals stored in the selected row of pixels are provided to areadout circuit 510. The pixels read from each of the columns can beread out sequentially using a horizontal addressing circuit 514.Differential pixel signals (V_(in) ⁺, V_(in) ⁻) are provided at theoutput of the readout circuit 510. The differential pixel signals areconverted to digital values by an A-to-D converter 512 having areference capacitor. This capacitor can be used to reduce the effectivecapacitance at the positive input signal node.

As shown in FIG. 6, the array 502 includes multiple columns 600 of CMOSactive pixel sensors 602. Each column includes multiple rows of sensors602. Signals from the active pixel sensors 602 in a particular columncan be read out to a readout circuit 604 associated with that column.Signals stored in the readout circuits 604 can be read to an outputstage 606. This output stage 606 is common to the entire array of pixels502. The analog output signals are sent to a differential A-to-Dconverter 608.

A further aspect of the A-to-D converter 700 is shown in FIG. 7. Anoffset signal is provided at the negative input signal node. In oneembodiment, the offset signal is generated by an offset adjustmentcircuit 702 to remove dark signals appearing on the pixel array 502. Inother embodiments, the offset signal electronically increases thebrightness of the image or compensates for some artificial offset addedin the signal processing chain in the readout circuit 510.

The offset adjustment circuit 702 includes two capacitors 704, 706. Alarger-valued capacitor 704 is connected between the negative inputsignal node 708 and ground. A smaller-valued capacitor is, in general, avariable capacitor 706. The top plate of the variable capacitor 706 isconnected to the negative input signal node 708. The bottom plate of thevariable capacitor 706 is connected either to a reference voltage or toground.

When a positive offset is desired during sampling, an Offset Enablesignal 710 is asserted. Otherwise, if a negative offset is desiredduring sampling, an Offset Clamp signal 712 is asserted. This signal isthen de-asserted to turn the clamp switch 716 off and turn the enableswitch 718 on. During conversion, if a positive offset is desired, anOffset Clamp signal 712 is asserted.

Other embodiments and variations are possible. For example, a variableoffset can be achieved by either using the variable capacitor 706 or avariable reference voltage 714. Further, the reset capacitor 704 can beomitted if the offset signal is relatively large compared to the fullinput voltage swing. Moreover, all references to voltages are forillustrative purposes only. The term “voltage” can be replaced with“current”, “power”, or “signal” where appropriate.

All these are intended to be encompassed by the following claims.

What is claimed is:
 1. An A-to-D converter system receiving power from at least two supply signals, comprising: a comparator having inputs, configured to compare an analog input signal with an adjustable reference level, said adjustable reference level generated from at least one of said supply signals; a logic circuit coupled to a first one of said comparator inputs, said logic circuit having a plurality of differently-sized capacitors and a plurality of switches, each capacitor connected to at least one switch, said at least one switch configured to successively connect said each capacitor to different levels of said supply signal; and an adjustable capacitor coupled between the first one of said comparator inputs and a first one of said supply signals, said capacitor configured to provide additional capacitance, said additional capacitance adjustable to change a full swing level of said adjustable reference level, such that the change enables said logic circuit to operate with said supply signals.
 2. The system of claim 1, further comprising: a plurality of latches to store digital values corresponding to said analog input signal when said comparator indicates a match between said analog input signal and said adjustable reference level.
 3. The system of claim 1, wherein said plurality of differently-sized capacitors are successively smaller by a factor of two.
 4. The system of claim 1, wherein said at least one switch includes p-channel and n-channel MOSFET switches.
 5. The system of claim 4, wherein said n-channel switch connects a ground-level supply signal to said each capacitor and p-channel switch connects another of said supply signals to said each capacitor.
 6. The system of claim 1, further comprising: an offset adjustment circuit coupled to the comparator, said offset adjustment circuit providing an offset to said analog input signal.
 7. A successive approximation A-to-D converter receiving power from at least two supply signals, comprising: a comparator having at least two inputs, to compare an input signal with a programmable reference level; and an adjustable capacitor coupled between one of said at least two comparator inputs and a first one of the supply signals, said adjustable capacitor providing a capacitive divider that changes a full swing of said programmable reference level, such that the change allows reference level programming to operate with a second one of the supply signals.
 8. The converter of claim 7, further comprising: a logic circuit coupled to said comparator, said logic circuit having successively smaller size capacitors, each capacitor connected to at least one switch, said at least one switch configured to successively connect said each capacitor to different ones of said supply signals.
 9. The converter of claim 8, wherein said successively smaller size capacitors are successively smaller by a factor of two.
 10. The converter of claim 8, wherein said at least one switch includes a p-channel MOSFET switch and an n-channel MOSFET switch.
 11. The converter of claim 10, wherein said n-channel switch connects a ground-level signal to said each capacitor and said p-channel switch connects a full-level supply signal to said each capacitor.
 12. The converter of claim 7, further comprising: a plurality of latches to store digital values corresponding to said input signal when said comparator indicates a match between said input signal and said programmable reference level.
 13. The converter of claim 7, further comprising: an offset adjustment circuit coupled to the comparator, said offset adjustment circuit providing an offset to said input signal.
 14. A method of converting analog signal to digital signal, comprising: adjusting a reference capacitor at an input signal node to change a full swing level of a reference level; selectively coupling conversion capacitors between the input signal node and a supply signal to program the reference level; comparing an input signal to said programmed reference level; and reading a digital output value if the comparison results in a match.
 15. The method of claim 14, wherein said adjusting said reference capacitor increases a total capacitance provided at the input signal node by said conversion capacitors.
 16. The method of claim 14, wherein said selectively coupling conversion capacitors includes programming said conversion capacitors to successively smaller digital value.
 17. The method of claim 14, wherein said selectively coupling conversion capacitors includes programming said conversion capacitors to successively larger digital value.
 18. The method of claim 14, wherein said comparison match occurs when said input signal and said programmed reference level is within some specified tolerance value.
 19. The method of claim 14, further comprising: providing an offset to the input signal.
 20. The system of claim 1 wherein the first one of the supply signals is a ground signal.
 21. The converter of claim 7 wherein the first one of the supply signals is a ground signal. 